The applications for flexible membranes in general include products such as sails, airfoil and wing systems, aircraft control surfaces, inflatable structures, airships, temporary shelters, liquid storage tanks, fuel tanks, flotation devices, seals and gaskets for aircraft surfaces, and door seals. Current materials in flexible membranes are based on the following techniques:
Core structural fiberous components are made of bonded scrim fiber groups. These core layers of flexible membrane designs are generally constructed with less than 10 yarns per inch in each of two orthogonal orientations. The structure of the core layer is generally formed by resin-bonded intersections between cross machine (CM) and machine direction (MD) yarns.
The MD and CM fibers provide along their thread lines or yarn directions the basic mechanical properties of elongation resistance and tensile strength. The control of elongation is important as this property allows the fabrication of structures that retain their designed shape over a range of loads. The modulus of a membrane material can be approximated by the elongation of fibers of that material under defined loads. Testing methods for measuring elongation follow ASTM (American Society for Testing and Materials) standards and use sample lengths up to 16 inches for testing accuracy.
These scrims are not generally of woven construction, and have very little structural integrity when the resin bonds have been broken. In short this type of bonded scrim has little durability with out modification and the addition of other components. In most systems these scrims deliver most of the fiber content necessary in CM and MD to control elongation and provide adequate tensile strength.
Fibers such as KEVLAR(tm) brand para-aramid, SPECTRA(tm) brand UHMW polyethylene, DYNEEMA(tm) brand UHMW polyethylene, Carbon, VECTRAN(tm) brand multifilament liquid crystal polymer, Zylon(r) polybenzoxazole (PBO), TECHNORA(tm) brand para-aramid, Polyester (Polyethyleneterephthalate) PEN (Polyethylene Naphthalate), TWARON(tm) brand para-aramid polymer, and Nylon polyamide fiber and polyester are all used in these core element scrims. (The applicant makes no claim to the trademarks.) Because of cost, larger yarn sizes are preferred. Most of these scrims use structural yarns of 1000 denier or larger. In some cases smaller non-structural yarns will be used in the opposing direction to provide for the bonding sites.
Polyester, PEN, Nylon, VECTRA(tm) brand polyster film, and other films are used for web stability. Because the scrim core layer does not provide off thread line stiffness, additional elements are used in current systems. In all current designs at least one layer of a stiff film (e.g. ½ mil polyester) is incorporated in the laminate. The film most commonly used is polyester, with a film thickness of from one quarter to one and two thousands of an inch. It should be noted that the addition of these films is not a means of adding a thermoplastic adhesive to the structure. These films are used to provide off-threadline mechanical properties and general mechanical durability.
All or most of the interconnect between MD and CD fibers in the core element or layer is adhesive or resin based. Because the core elements are not generally woven the integrity of these systems is based on the various resin adhesive bonds in the assembly. These resin adhesive systems are typically crosslinked elastoners or other crosslinked adhesive resins. Yarn is bonded to yarn and yarn is bonded to film. The result of this dependence on film interlayer and adhesive is that these structures have overall durability that is limited to the properties of the film and the adhesives used. The low to no twist yarn used in these scrims contribute to these adhesive failures.
Because of the limitations of film mechanical properties, off axis fiber components have been developed. Some products add low count, less than 15 ends per inch (epi) structures or elements on thread lines that are off or non-aligned with the 0 to 90 degree angle between the MD and CM axis. As with the MD and CM scrims these yarn layers are coarse, low-end count structures. Again, like the core scrims, the off axis scrims are not woven and at most contain crossing points only in one direction. Also like the core scrims these off axis scrims comprise yarns of little or no twist. Twist is a secondary process and adds cost.
Because of the limitations of the non-woven core, film and off axis elements, some systems include woven cover fabrics bonded to the outside of the system. These wovens may contain all the yarn types of the core elements. However most cover fabrics use deniers much smaller than 1000. In all current products the cover fabric is bonded to the core materials with its MD and CM at 0 and 90 degrees to the core elements. Only laid or bonded scrims are placed off axis.
Simple coated and saturated fabrics are also used for flexible membrane applications. In these designs there are no scrim elements. However films may be attached by adhesive bonding.
Resin bonded membrane systems have a catastrophic failure mode. Because there is little woven interlock in the structural fiber elements of the current systems, the structure can fail without failure of the fiber. Low and no twist fiber contributes to this result. These structures can delaminate and the fiber separate without breakage of fiber. In flex and tear mode the potential performance of the fiber is not realized unless the adhesive bonding quality is equal to the fiber strength. In practice this is not possible, so adhesive bonding failures are a cause for premature and catastrophic failures.
Resin and films have limited properties relative to fiber. Because of the high dependence on resin bonding, current products are limited in durability to the flex and adhesion of the bonding mechanisms.
The cosmetics of these products may suffer from local delaminating and mildew prior to a failure in performance. Because the core and off axis fiber layers use larger yarns, there tend to be void spaces or interstices in the fabric composite. When moisture enters these void spaces between the films, delaminating and mildew are frequently the result.